The present invention relates to a vulcanized rubber for a heat resistant vibration-isolator which has an excellent durability and processability, and a low dynamic spring constant.
Superior characteristics in durability, thermal stability, dynamic spring constant and the like are required for a vibration-isolator rubber to be widely used in the automobile field and the like. The dynamic spring constant is the extent of the change of elastic modulus (dynamic shear modulus of elasticity) at a high-frequency vibration and is represented by the ratio of dynamic shear modulus of elasticity and static shear modulus of elasticity. A dynamic spring constant is usually larger than 1, the lower the dynamic spring constant is, the superior the vibration-isolator property.
Hitherto, it has been well known that ethylene-.alpha.-olefin-non-conjugated diene copolymer rubber is used as rubber material for the vibration-isolator rubbers, but that this copolymer is inferior in durability although it is superior in thermal stability.
On the other hand, vulcanized rubber with a high molecular weight polymer (its intrinsic viscosity [.eta.] measured at 70.degree. C. in xylene is usually 2.5 dl/g or more) shows an improved durability, but does not have a satisfactory dynamic spring constant. Such rubber still has a problem that in the processing before vulcanization, it is apt to exhibit so-called "bagging" wherein a rubber composition rises to the surface of a roller during roll processing.
When a softener or carbon black is added in large amounts to the high molecular weight polymer in order to improve the processability, the dynamic spring constant of the resulting rubber increases and hence its vibration-isolator property becomes inferior.
The vulcanized rubber obtained from a rubber composition which comprises ethylene-.alpha.-olefin-non-conjugated diene copolymer rubber and hydrogenated liquid diene polymer (Japanese Examined Patent Publication (Koukoku) NO.3-73584) is excellent in processability, but suffers from a poor dynamic spring constant.
With a view to improve both the processability and vibration-isolator property of a vulcanized rubber, there has been proposed in Japanese Unexamined Patent Publication (Koukai) NO.53-22551 that a low molecular weight EPDM is blended with a high molecular weight EPDM. But, in this method, the dynamic spring constant of the resulting rubber is not satisfactory though the processability and durability can surely be improved. In this method, vulcanized rubbers with an excellent durability and processability, and a low dynamic spring constant can hardly be obtained.
Under the circumstances, the present inventors made an intensive study in order to obtain a vulcanized rubber for heat resistant vibration-isolator with a superior durability and a low dynamic spring constant, and, as a result, found that a vulcanized rubber with such superior properties such as excellent thermal stability, superior durability, excellent processability and low dynamic spring constant can be obtained when a rubber composition comprising a copolymer rubber having a particular intrinsic viscosity and a liquid diene polymer having a particular melt viscosity is vulcanized with an organic peroxide.